1. Field of the Invention
This invention relates to photoresists formed by polymerization of certain substituted butadiene monomers. The invention is especially useful in microlithography, particularly for producing semiconductor devices on silicon chips.
2. Description of the Related Art
The use of anionically or zwitterionically polymerisable monomers as resist materials for microlithography is known in the art, as discussed in U.S. Pat. No. 4,675,273 Woods et al and U.S. Pat. No. 4,675,270 Woods et al both assigned to Loctite (Ireland) Limited, the contents of which are incorporated herein by reference. Previous methods involved spin coating a solution of a cyanoacrylate polymer onto a substrate. U.S. Pat. No. 4,675,273 describes a method for applying a polymeric resist coating to an etchable substrate which comprises exposing the substrate to be coated to the vapour of an anionically polymerizable monomer of the formula: EQU CH.dbd.CXY I
where X and Y are strong electron withdrawing groups and R is H or, provided that X and Y are both --CN, R may be C.sub.1 -C.sub.4 alkyl, for sufficient time to deposit a polymerizable coating thereon. Particularly preferred monomers are 2-cyanoacrylate esters. The coated substrate is subsequently imaged using high energy radiation; the image is developed by conventional solvent development processes; the image is etched using a suitable plasma or acid etching process; and the resist coating may be subsequently removed by heating the coating to a temperature above the polymer depolymerization temperature.
U.S. Pat. No. 4,675,270 describes an imaging method which comprises
(a) providing a substrate having a surface reactive to activate polymerization of a monomer defined by the formula I as defined above; (b) treating the surface of the substrate with a photosensitive compound which releases an acid when exposed to actinic or ionizing radiation; (c) subsequently imagewise exposing the substrate to radiation of an energy effective to release said acid from said photosensitive compound; and then (d) exposing the substrate to vapours of one of said monomers for sufficient time to form a polymer coating over the substrate in the areas thereof not exposed to the radiation, forming a relief image.
Cyanoacrylate polymers generally form positively imaged resists i.e. the relief image is in areas which have not been exposed to radiation (see U.S. Pat. No. 4,279,984 Matsuda et al, assigned to Matsushita Electric Industrial Co. Ltd.). The process of U.S. Pat. No. 4,675,270 also forms a positively imaged resist.
It is also known to form negatively imaged resists i.e. the relief image is in areas which have been exposed to radiation. U.S. Pat. No. 4,551,418 Hult et al describes a process for generating a negative tone resist image comprising the steps of:
(1) coating a substrate with a film that contains a cationic photoinitiator; (2) exposing the film in an imagewise fashion to radiation and thereby generating cationic initiator in the exposed regions of the film; (3) treating the exposed film with a cationic-sensitive monomer to form a film of polymer resistant to plasma etching; and (4) developing the resist image by etching with a plasma.
It is an object of the present invention to generate negatively-imaged resists using anionic or zwitterionic polymerizable monomers. 1,1-disubstituted 1,3-butadienes are already known. U.S. Pat. No. 3,316,227 Gerber assigned to Lord Corporation describes 1,1-disubstituted diunsaturated compounds having a formula selected from the groups consisting of ##STR3## where R.sup.1 is selected from the group consisting of hydrogen, alkyl groups containing from 1 to 5 carbon atoms, phenyl and halogen; where R.sup.2 is selected from the group consisting of hydrogen and methyl, and where X and Y are dissimilar electron-withdrawing groups and are separately selected from the group consisting of cyano, carbethoxy, ethyl sulfone, phenyl sulfone, formyl, acetyl, benzoyl, diethyl, phosphonyl, amide and phenyl. These compounds are described as having utility in the fields of adhesives and coatings.
U.S. Pat. No. 3,554,990 Quinn et al assigned to Eastman Kodak Company describes esters of 2-cyanopenta-2,4-dienoic acid having the structural formula: ##STR4## wherein R.sup.3 is an alkenyl group of 2-10 carbon atoms or an alkoxy substituted alkyl group of 2-10 carbon atoms. These esters are said to be useful as adhesives for general and particularly for surgical uses.
There is no suggestion in the prior art that polymers of substituted 1,3-butadienes might find utility as photoresists.